CDMA (Code Division Multiple Access) system using spread spectrum technology as a multiple access method in which a plurality of communication apparatuses communicate mutually have been researched broadly, and practical use has been reached. The major advantage of the CDMA system is the ability to accommodate a large number of users because it can be designed to have a high-level of security, interference resistance and superior frequency efficiency. In addition, there is an increasing demand for transmitting a large amount of data such as image data, etc. within a short time in a downlink channel, with diversification of the recent services in the mobile communication field.
Under the background of 3GPP (Third Generation Partnership Project) which is one of the standard organizations relating to a CDMA system, use of DSCH as a common channel in high-speed data transmission of the downlink channel is under consideration. The DSCH is used in a cellular system as shown below. That is to say, abase station apparatus controls the transmission formats such as modulation method, transmission rate, user assignment, etc. of a DSCH channel every one frame, and the data is transmitted to a communication terminal apparatus. The control contents of such transmission formats are notified to a communication terminal apparatus using TFCI (Transmit Format Combination Indicator) included in DPCH which is assigned individually to each communication terminal apparatus. The communication terminal apparatus learns the transmission formats of the DSCH with reference to this TFCI, and can obtain the received data after demodulating the data of DSCH. Hence, those DSCH and DPCH are code-multiplexed and then transmitted.
If a plurality of channels are code-multiplexed as described above, there is a problem that the signal to a target user could interfere the other users. The transmission power control, in which the effect of interference to other stations is reduced by adaptively changing the transmission power in the transmitting side so that an optimum reception level can be obtained in the receiving side, is widely known as a technology to reduce the interference to other stations.
Among such a transmission power control, a closed-loop transmission power control in which a TPC (Transmit Power Control) command to instruct an increase/decrease of transmission power so that the SIR (Signal to Interference Ratio) which is measured in the communication terminal apparatus based on the receiving signal of the downlink channel approaches a predetermined desired SIR (hereinafter, it is referred to as “desired SIR”) is generated, and the increase/decrease of the transmission power based on the TPC command, is performed in the base station apparatus.
However, if DSCH and DPCH are multiplexed together, there is a problem that DSCH could be an interference to DPCH, and the transmission power control cannot be appropriately carried out in the DPCH. In other words, since a packet is transmitted in burst in DSCH, an interference level is rapidly fluctuated in the period where the packet is inserted as well as in the period where the packet is not inserted, and consequently, there is a problem that the transmission power control in DPCH cannot follow the rapid fluctuation of such an interference.